Controlling bulk particulate flow rates

ABSTRACT

A method of transferring particulates from a storage silo to another container or to a process, in which the particulates are fed from the silo into a transfer device comprising a conduit having a first convergent section ( 2 ) converging in the direction of particulate flow followed by a second convergent section ( 3 ) also converging in the direction of particulate flow. The convergence geometry is arranged so that the particulates passing though the conduit under gravity are discharged from the outlet of the second convergent section in a vena contracta flow pattern. The vena contracta effect provides a structured outlet flow where a portion of the outlet flow is narrower than the outlet itself and this enables high flow rate filling of vessels or containers through relatively small apertures (e.g. tanker hatches).

FIELD OF INVENTION

The present invention relates to a method and apparatus for controlling or manipulating the flow rates of particulates. The invention has application to controlled filling of vessels and other containers used in industry, or dosing into processes. The invention provides a transfer device used to direct particulates from a storage vessel to another vessel or container for transport or for utilisation in a process.

BACKGROUND OF THE INVENTION

The discharge problems that are experienced in industry can be categorised into two main groups, these being flow irregularities and flow stoppages. Also there are two basic flow patterns that are commonly encountered in industry; these being core flow and mass flow.

Core flow discharge is the most frequently encountered flow pattern in industry and is characterised by the development of a preferential flow channel in the particulate material that extends from the outlet to the top surface of the vessel. This type of pattern gives a “first in—last out” discharge of the material within the vessel, and in instances where the vessel is replenished prior to complete discharge (which is often the case in industry) the situation can arise whereby the material in the lower section of the vessel may not be finally discharged for a substantial period of time. The geometry of core flow vessels typically has its roots in history rather than any scientific basis

By contrast, mass flow vessels are invariably “bespoke” designs that have their convergent angles, wall finishes and outlet sizes defined by data obtained from shear cell measurements to determine the flow characteristics of the bulk solid. The flow pattern obtained in mass flow vessels is such that the shear stress between the bulk solid and the wall is less than the internal strength of the material, resulting in a flow occurring along the wall. This flow behaviour results in a complete displacement of the inventory; even when relatively small quantities of material are discharged. “First in—first out” discharge of material is achieved.

It is known that for a given outlet size (provided that it is beyond the critical size for the initiation of flow) a nominal flow rate of particulates will result, and that changes in the area of the outlet will bring corresponding changes in flow rate.

For gravity discharge systems, techniques for achieving this change in outlet area usually take the form of surfaces that are introduced into the region of the outlet such that the flowing stream of particulates impinges against the surfaces. This impingement surface is most commonly introduced perpendicular to the direction of flow.

Conventional flow control arrangements utilise specific geometries for the approach to the outlet, at which point an impingement type flow restriction is applied.

Both design features invariably generate regions of particulate that do not flow during the operation of the equipment. These static regions of particulates define the boundaries of the flow channel and represent high effective friction surfaces that can contribute to a decrease in discharge reliability, consistency and instantaneous consistency.

Increases in flow rate (particularly for cohesive or poorly flowing particulates) can be achieved through the introduction of air at points on the convergent surfaces of vessels. The quantity and control of air volumes is invariably fixed at an arbitrary value; resulting in poor flow control and high levels of air entrainment (leading to poor filling efficiency and the potential for high levels of fugitive particles in the vicinity of the discharge operation).

SUMMARY OF THE INVENTION

The present invention provides an improved approach for controlling the flow rates of particulates using a mass flow discharge system which enables a stable vena contracta effect to be established in the discharging bulk particulates in the vicinity of the outlet.

This effect is achieved through the use of geometries and flow channel boundary surfaces in a transfer conduit that bring about a reduction in flow channel area without generating excessive effective friction (and the resultant loss of bulk particulate shear against the bounding surfaces).

Accordingly in its broadest aspect the present invention provides a method of transferring particulates from a storage silo to another container or to a process, in which the particulates are fed from the silo into a transfer device comprising a conduit having a first convergent section converging in the direction of particulate flow followed by a second convergent section converging in the direction of particulate flow, and the convergence of these sections or of the second section is arranged so that the particulates passing though the conduit under gravity are discharged from the second convergent section as a vena contracta.

The vena contracta discharge flow is directed into the other container or used as a feed into a process.

The conduit for transfer of the particulates may have a first convergent section including first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of the second convergent section, and in the first convergent section the first pair of opposed wall surfaces converge towards each other in the direction of flow, the second pair being convergent, parallel or divergent, whereas in the second convergent section the second pair of opposed wall surfaces converge towards each other in the direction of flow, the first pair being convergent, parallel or divergent.

In another configuration, the first convergent section includes at least two opposed wall surfaces which converge towards each other in the direction of flow to form a slot outlet at the junction between the first convergent section and the second convergent section, and the second convergent section includes at least two opposed wall surfaces which converge towards each other in the direction of flow to form a slot outlet for discharge of particulates, the two slot outlets extending in respective directions that are substantially normal to each other.

In the two conduit configurations given above, suitably it is the geometry of the second convergent section which is arranged so that the flow discharge of the intended particulates takes the form of a vena contracta.

Preferably the conduit includes an inlet section upstream of the first convergent section, to direct the particulates into the first convergent section. The inlet section may have parallel walls.

The conduits found suitable to provide a vena contracta effect form a further aspect of the present invention.

Therefore, according to another aspect of the invention, there is provided a device for gravity transfer of bulk particulates comprising a first convergent section including first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of a second convergent section, and in the first convergent section the first pair of opposed wall surfaces converge towards each other in the direction of flow, the second pair being convergent, parallel or divergent, whereas in the second convergent section the second pair of opposed wall surfaces converge towards each other in the direction of flow, the first pair being convergent, parallel or divergent.

According to a further aspect, a device of the invention has a first convergent section in which at least two opposed wall surfaces converge towards each other in the direction of flow to form a slot outlet at the junction between the first convergent section and a second convergent section, in which at least two opposed wall surfaces converge towards each other in the direction of flow to form a slot outlet for discharge of particulates from the outlet section, the two slot outlets extending in respective directions that are substantially normal to each other.

Typically a device of the invention is an open-ended conduit having an inlet section, a first convergent section and a second convergent section, in which in use the inlet section is uppermost and through which particulates pass from the inlet section to the first and second convergent sections.

The two geometries set out above as two separate aspects may be combined, so that in the first aspect both the first convergent section and the second convergent section converge in the direction of flow to form a slot outlet and the two slot outlets extend in respective directions that are substantially normal to each other.

Using these basic geometries, the present invention provides a device primarily for high flow rate transfer, in which the geometry of the outlet section can arranged so that for the intended particulates the flow discharge takes the form of a vena contracts.

The properties of any specific particulate material and the dimensions of the source silo and delivery point will have a bearing on the dimensions and convergent angles of a transfer device for use in the process of the present invention. The following detailed description of various embodiments of the invention will assist in understanding the underlying principles and the way that the geometry of the system can be arranged to achieve the desired effect, so as to put the invention into practice.

DESCRIPTION OF THE DRAWINGS

FIG. 1 provides schematic side views of situations in which the present invention can be put to use: FIG. 1A in tanker filling operations; FIG. 1B in packaging/dosing operations;

FIG. 2 is a schematic side elevation of an example of an apparatus of the present invention;

FIG. 3 is a schematic end elevation of the apparatus of FIG. 1;

FIG. 4 illustrates test equipment for assessing core flow parameters;

FIG. 5 illustrates test equipment for assessing mass flow parameters;

FIG. 6 illustrates several variations of the outlet aperture of the test equipment for mass flow;

FIG. 7 illustrates aspects of the cross-sectional area of the flow stream in the secondary convergent section when the convergency provides 80% aperture reduction, showing: FIG. 7 (a) flow regions present in horizontal cross-section; FIG. 7( b) velocity profile along main axis of outlet; FIG. 7 (c) air induction path and influence over void pressure.

FIGS. 8 a,b,c illustrate the same aspects as FIG. 7 a,b,c, in a system where the convergence provides 60% aperture reduction;

FIG. 9 a,b,c illustrate the same aspects of FIG. 7 a,b,c, in a system where the convergence provides 40% aperture reduction.

DETAILED DESCRIPTION OF THE INVENTION

The invention can be used to increase flow rates over those commonly associated with a given outlet area. The basic geometry ensures that particulate flow will occur at the rigid (non-particulate) surfaces defining the flow channel and facilitates the induction of air into the outlet region to enable a stable vena contracta effect to be established in the discharging bulk particulates in the vicinity of the outlet.

The form of vena contracta achieved by the present invention is shown in FIGS. 7 a and 8 a. Under appropriate flow conditions, achievable using the present invention, the discharge assumes a stable dynamic structure with a characteristic configuration shown in FIGS. 7 a and 8 a, in which the transverse dimension of the discharge flow is less than the corresponding dimensions of the outlet from which the flow is issuing—see area B.

Typical situations in which the invention may be used are shown in FIGS. 1A and 1B. FIG. 1A shows filling a bulk handling tanker (20) from a storage silo (30). A flow control or transfer device (40) of this invention receives particulates exiting from the silo (30) via a delivery chute (31) and provides a controlled and efficient delivery of the particulates into the relatively narrow entry hatches (21) of the tanker (20).

In another application, for packaging or dosing of the invention shown in FIG. 1B, particulates are fed from storage vessels (35) via delivery belts (38) into a buffer vessel (37). Then a device (45) of this invention directs the particulates into containers (22) for the particulates, or into containers or vessels containing materials requiring a dose of the particulates.

The device by which the desired vena contracta discharge flow can be achieved is illustrated by reference to the structure shown in FIGS. 2 and 3. As already shown in FIGS. 1A and 1B, a device of the invention can be used for manipulating and controlling the flow rate of bulk particulates (6) in transfer from a storage vessel (not shown in FIGS. 2 and 3) to another vessel or container (not shown), or to a hopper (not shown) that provides feed to a process. The invention uses an arrangement of geometries and surfaces (4, 5, 7, 8, 9, and 10) and the interaction of these features such that the flow rate of particles is enhanced. Such enhancement takes the form of increased flow rate and/or increased consistency of instantaneous flow rate.

The device takes the form of an arrangement of surfaces (7,8,9,10) that define the boundary of the moving bed of particles, and which at the outlet section (4,5) of the device can facilitate the development of a vena contracta of flowing particles and an upward induction path for gas. The combination of flow channel boundaries that are defined by geometrically appropriate surfaces, convergent shape and resultant gas induction, combine to support a stable vena contracta of the flowing stream of particles. Manipulation of aforementioned geometries, gas induction pathways and gas volume (at either the design or operational stages) provide a control function over flow rate of particulates. The establishment of the vena contract effect, and the resultant structured outlet flow where a portion of the outlet flow is narrower than the outlet itself, enables high flow rate filling of vessels or containers through relatively small apertures (e.g. tanker hatches, or other small vessel entrance ports).

In FIGS. 2 and 3, there is shown a flow stream of bulk particulates 6 which may be entering the device directly from a vessel or from an indirect flow path (i.e. chute), as shown in FIGS. 1A,1B. The uppermost inlet section 1 of the device may have the cross-sectional form of a square, circle, rectangle or other shape, and should ideally be devoid of ledge forming intrusive features.

Bulk particulates 6 flowing through inlet section 1 reaches the first convergent section (2), whose shape is formed by the inter-relationship of opposed pairs of walls (7,8). Walls (7) are convergent along their length, whilst walls (8) can be convergent, vertical or dilatant. The geometry of the opposed walls (7,8) must be designed to enable shear of bulk particulates against their internal surfaces when operating with the bulk particulate in a packed bed condition. The inter-action of walls (7) and (8) preferably forms a slot outlet. The inclusion of radii or fillets along the junctions of walls (7) and (8) is highly desirable to assist flow.

Beneath section (2) providing the first convergent section, the second convergent section (3) is formed providing an outlet section by the inter-action of opposed pairs of walls (9) and (10). The angle for walls (9) must be conducive to bulk particulate shear against their internal surfaces and walls (10) can be convergent, vertical or divergent. The angle of walls (9) combined with the depth and angle of walls (10) define the outlet proportions (4) and (5).

For high bulk particulate flow rate applications, the relationship between the outlet proportions of the first convergent section (2) and the interaction between walls (9) and (10) can be used to establish a stable vena contracta of flowing bulk particulates at the entry into the second convergent section (3). In this operational configuration the inter-relationship between walls (9) and (10) allows the establishment of a directional induction of gas into the outlet area of the first convergent section (2). The proportions of walls (9) and (10) may be modified to change the “bell mouth” induction effect to suit different operational requirements.

Supplementary gas may be introduced or extracted from the first convergent 2 or its vicinity to facilitate the dilation of flowing bulk particulates in this region.

The invention was achieved following a case study involving observation and adjustment of existing apparatus and subsequent experimental work, as discussed below. This review of the inventor's studies will provide guidance for the implementation of the invention in other circumstances.

Case Study

In a silo for powdered coal carefully designed to give mass flow on discharge of the coal, it was found that the discharge reverted to core flow as a direct result of shear generated by the intrusion to flow imposed by clamshell doors closing the outlet at the base of the silo. Additionally, it was found that, although the use of beak doors gave very high flow rates, little fine control was possible until relatively small apertures were reached. The problem was resolved by the use of internal liners to re-define the aperture within the discharge section of the silo, which gave good controllability over a wide range of apertures. In effect, the converging section presented by the internal liners performed the same function as the beak doors, but at a point higher within the silo. Hence, the convergent liners impinged upon the material before a free fall condition was reached (as was the case when the material reached the beak doors). It was also observed that air enhanced dilation of the material as a result of air being drawn in along the gap created between the tops of the beak doors and the body of the hopper outlet provided a beneficial effect.

In the apparatus studied, the techniques used to control the flow were options frequently used in industry, having been supplied “off the shelf” and thereby seen as “state of the art” approaches.

The findings and implications of the case study indicated that the primary requirements could be met by the use of a double plane flow system, providing a primary plane flow hopper beneath which a secondary plane flow hopper section is installed whose axis of discharge is perpendicular to that of the hopper above.

Experimental work with this system indicated that in situations exemplified by the conditions of the case study, advantageous effects are obtained when the arrangement of the second section is such that a vena contracta discharge pattern is achieved. The arrangements which favour this effect are discussed below.

Development Based on Case Study

In order to evaluate the double plane flow approach for a low shear dispensing head design, a test rig was set up to incorporate several design features aimed at imposing control over the rate of material dilation and hence flow rate.

Variable Geometry Primary Plane Flow Liners

The primary plane flow section was constructed with adjustable liner plates, such that the geometrical requirements for mass flow for the test material could be accommodated. The liners were arranged such that their pivots were at the base of the liner (i.e. at the outlet). By adopting this design feature, any adjustments to the convergent angle would not affect the outlet dimension in use (as would be the case if the liners were pivoted along their top edge) or any position.

Dilation Section in Primary Plane Flow Sections

The cross-sectional area of the dispensing head was increased at the transition at the start of the primary plane flow section. This design feature was incorporated to allow the bulk solid to dilate and thereby reduce the internal strength of the material—hence offering a degree of “flow assistance” for more cohesive particulates.

Air Induction Ports

The provision of air inlets at the top of the primary dilation section gave scope for influencing the air pressures at the point of dilation. By influencing the level of interstitial air pressures, the discharge rate can be influenced.

Test Programme

In order to evaluate more fully the impact of flow channel development on discharge consistency in a directly comparable system, a test programme was initiated comparing flow stability for core and mass flow outlet control regimes.

Core Flow Test Equipment

In order to ensure that ledges of retained material would develop and be maintained through the test programme a flat-bottomed discharge section was constructed (see FIG. 4). A central outlet slot was cut (60 mm×160 mm) and slide gates installed such that the length of the slot for each test condition could be adjusted by inserting or retracting the two gates. The top plate of the discharge section was constructed to enable attachment to the mid section of the existing test rig. Initiation of discharge was achieved by the removal of a blanking cover under the outlet.

Mass Flow Test Equipment

The counterpart to the core flow equipment, described in the previous section, was a dispensing unit designed such that mass flow could develop during operation (see FIG. 5). Changes in outlet dimension were effected by the complete replacement of the secondary plane flow section with a unit having a more abbreviated convergent section. Four different convergent sections were used during the trials—these gave apertures which were 20%, 40%, 60% and 80% of the outlet from the first convergent flow sections (see FIG. 6).

Bulk Particulate

Discharge trials were undertaken using 500 kg of detergent powder. This material was selected for use as it exhibited sufficient free-flowing characteristics that it could reasonably be expected to discharge from the 20% outlet apertures that would be investigated, whilst still offering a degree of air impermeability.

Test Procedure

For each of the outlet configurations under investigation, the loss in weight from an upper feed hopper discharging into the test assembly was monitored using a data logger, whilst simultaneously recording the signal output from the instantaneous mass flow rate meter. The former set of data was used to determine the mass flow rate of bulk particulate, whilst the latter set of data was used as an indication of the consistency of the discharge stream from the test section.

Test Results

The sets of data obtained from the discharge trials with core and mass flow equipment were processed and examined to yield comparative data to highlight the effects of impeded flow channel development in an industrial scale laboratory experiment.

Discharge Rates

Examination of the discharge rate test data indicated that at 20% apertures the average discharge rate for core flow was higher than for mass flow by 17%. This result correlates with the general model of core flow yielding higher discharge rates due to the improved ingress of air into the bulk solid by virtue of the dilated bulk particulate conditions present within the flowing preferential channel (i.e. a narrow highly dilated flow channel offers a path of least resistance to air ingress compared to a mass flow system where no such preferential channel exists). At 40% apertures, the discharge rate of the mass flow system overtakes the core flow equipment and develops a 13% higher discharge rate (based on averaged values). Data obtained from the 60% aperture is of particular interest, as it suggests a higher discharge rate is possible with the mass flow system at this aperture than can be achieved at 80%. This set of data bears a striking resemblance to that obtained for powdered coal in the first case study mentioned above.

The apparent similarity between the discharge behaviour for the detergent at 60% aperture and the coal at 60% aperture, is interesting in that for the former test condition the reduction in outlet area was obtained through shortening the length of the slot outlet, whilst in the latter test reduction in outlet area was arrived at by altering the width of the slot only using beak doors.

Photographs taken during discharge trials with the detergent powder at 60% aperture showed a very clearly defined vena contracta effect.

The inventor suggests a model whereby at large secondary plane flow section apertures, the main influence over the discharge capability of the system lies in the ability of air to be induced into the dilating bulk particulate present at the start of secondary plane flow section. The geometry of the secondary plane flow section is such that the vertical sides offer minimal contact opportunity for the discharging material (which will already have a vector imparted into its passage from the main outlet by virtue of the shearing contact with the main convergent section walls), whilst the convergents that form the end walls of the slot outlet will be in contact with the discharging material descending from the vertical end walls of the main plane flow section. Increased air induction along the main axis encourages a higher dilation rate and supports the establishment of the vena contracta effect.

FIG. 7 a illustrates the cross-sectional area of the flow stream at 80% aperture ie the convergent of the section results in a discharge aperture that has an area that is 80% of the of inlet area of the section. In this illustration the regions of material influenced by contact with the convergent end walls of the secondary plane flow section are annotated “A”, whilst the region whose flow behaviour is most significantly influenced by the air induction and establishment of the vena contracta effect is annotated “B”. A graph based on the inventor's model, and showing the variation in particle velocity along the centreline of the secondary outlet is also shown (note that this graph is for illustrative purposes only since no physical measurements of particle velocity across the discharge stream were undertaken).

FIG. 7 b serves to augment the flow model shown in FIG. 7 a, and serves to show the influence of the secondary section end wall convergent on the development of the discharge characteristic of the particulates. This illustration proposes a flow system whereby the deceleration of a bed of particles coming into packed bed contact with the convergent end walls establishes a velocity profile in the bulk particulate stream that extends to a given depth before the higher particle velocities present in the body of material dominates the system behaviour.

FIG. 7 c illustrates the possible void pressure distribution that is established within the convergent section upon discharge, relative to a given vertical height within the test equipment. The cross-sectional sketch of the test equipment (on the left side of FIG. 7 c) serves to show the proposed velocity profile across the centreline of the test equipment. Higher velocities (as indicated by larger directional arrows) are suggested to be found at the walls of the primary discharge section in the immediate vicinity of the start of the secondary section.

Because the primary convergent section operates in mass flow, shear of bulk particulates against the bounding wall surfaces develops. The vertical end walls of the primary plane flow section offer very low effective friction, whilst the convergent along the main axis of the outlet present the most significant influence over discharge rate. The potential for the development of relatively high particle velocities at the ends of the outlet is prevented by the “backing up” of the particle bed that is contact with the secondary section end wall convergents. However, the presence of a vertical wall in the form of the secondary section along the main axis of the primary outlet provides a mono directional route (i.e. the inducted air is drawn in vertically by the channelling effect of the vertical walls), which aids penetration into the bed of particles in the vicinity of the primary outlet. It is believed that this air induction along both sides of the main outlet axis serves to boost discharge rates in two ways:

-   -   a) Firstly, air inducted into the region of particle/wall shear         serves to dissipate void vacuum more rapidly at this         point—giving rise to increased dilation at the wall and hence         lower wall friction.     -   b) Secondly, the volumes of air being inducted are unknown—but         it is highly likely that air penetration will not be restricted         to the preferential channel developed by shear at the wall, but         will also penetrate to a depth inwards from the wall. Thus the         bulk properties towards the wall are also likely to exhibit a         more dilated state (and hence lower internal friction).

Both of these phenomena will alter the bulk properties of the particulate from those determined through conventional shear cell testing (and upon which classic vessel design is based).

Issues to be resolved are whether the establishment of the vena contracta (with its associated high rates of dilation and hence rate of change in void pressure relief) induct air into the system in sufficient volumes to boost flow, or whether the air drawn into the outlet region permits a directional induction of air with sufficient velocity (and hence penetrative capability through the dilating material) to establish a vena contracts.

Relating to FIG. 8 a (60% aperture), the inventor's model considers an increased depth of zone “A” that results from the greater duration of material contact with the secondary section end walls (i.e. the length of the end walls is greater than that for the 80% case—compare FIG. 8 b and FIG. 7 b). This increased depth of shear velocity profile at the ends of the slot outlet provides a compaction at either end of the already plastically deformed bed of discharging material from the primary plane flow section. As a result of this deformation of the flow channel, a deepening of the vena contract effect at the outlet develops (indicated by “B” in FIG. 8 a). Again, the precise determination as to which phenomenon predominates the development of this channel profile is difficult to arrive at—there being a high degree of co-dependency—but the observable effect is that a significant deepening of the channel is established that serves to provide a larger conduit for inducted air to pass into the primary plane flow section (the passage of this air flow may also aid the self sustaining nature of the “necking” profile of the flow channel at the outlet).

At smaller apertures (less than 60% opening) the influence of the slot length (and the vertical element that this represents) diminishes to a point whereby the material in contact with the end wall convergents interact across the secondary plane flow slot length and decelerate the discharge stream to a point whereby they generate a conventional packed bed condition. With these conditions in place, the outlet of the secondary plane flow section exerts the primary influence over achievable discharge rate.

Considering FIG. 9 a (illustrating conditions at 40% aperture) the length of the convergent end walls, and the resulting depth of shear profile through the particulate solid at these regions coincide to choke the flow of material to the extent that flow becomes stabilised as a moving packed bed. Under these conditions the flow rate becomes subject to conventional models of behaviour and the outlet area becomes the controlling factor with respect to achievable discharge rates.

Following these observations and the formulation of a likely model for the flow behaviour that had been generated, consideration was given as to how the establishment of the vena contracta of the discharge stream could be influenced to yield a more predictable discharge rate over the range of outlet apertures. It was considered that the most effective approach could be the application of air into the bulk particulate at the start of the primary convergent.

Air was introduced through six feed pipes (three equally spaced on opposing sides of the primary plane flow test section). The volume of air introduced into the test section was set to a volumetric rate of approximately 3% (this volume being the maximum available from the air mover used during the test programme) of the previously recorded average volumetric flow rate of the bulk particulate through the test rig when operating in mass flow.

Repeat discharge trials with the addition of air into the primary plane flow section were undertaken. During the trials it was apparent that the stream of material exiting the test section was more stable and consistent.

During trials undertaken at 80% aperture, the vena contracta effect that was evident during the initial trials was eliminated and increased flow stability was apparent in the discharge stream.

The inventor considers that the elimination of the vena contracta effect was caused by the partial dissipation of void pressures in the bulk particulate. By virtue of the reduced bulk void pressure to be relieved at the outlet, the draw of air into the secondary plane flow section was significantly reduced—resulting in minimal “up draught” to support the flank distortion of the flow channel. Observations of the discharge trials at 60% with air injection showed that a small vena contracta effect was evident—suggesting that some void vacuum was still evident. It is reasonable to assume that the optimal volume of air injection for a given bulk solid would be reached at the point whereby no air induction (i.e. stream deformation) along the flanks is evident. The author speculates that this optimum volume of air injection could be well below 10% by volume of the particulate being handled—for a given discharge rate.

Examination of the data obtained from the data logger showed that increases in discharge rate of 77% and 55% had been achieved over the two discharge trials undertaken using additional air.

Trials undertaken at 60% aperture showed smaller improvements in discharge rate (31% and 17%), but the almost total elimination of the vena contracta in the discharge stream was evident.

The data obtained from the air introduction trials undertaken at 60% apertures, showed a small improvement in overall discharge rate. The increase in flow rate was considerably less than that achieved at 80% apertures. At apertures less than 60% the introduction of air at 3% by volume of the discharge rate had a negligible effect on discharge rates.

The inventor considers that the minimal improvement in discharge rates at small apertures can be attributed to the low rates of dilation present adjacent to the air induction ports. This combination of relatively low bulk particulate permeability with the low air supply pressure is likely to have reduced the potential for significant influence over flow characteristics to become established. However, at higher rates of dilation associated with the 60% and 80% trials, sufficient voidage (and hence increased permeability) would develop that the introduced air could dissipate more efficiently into the bulk particulate.

These findings pose an interesting possibility, in that an inverse relationship may exist between optimised air volumetric flow rates and discharge rate. If this were to be proven to be the case in larger scale systems, the implications for industry could be very beneficial, in terms of reduced energy consumption (less air requirements) and reduced dust generation/spillage as a result of over-aerated bulk particulates. Having considered the effects of air introduction into a double plane flow test section from a discharge rate perspective, the impact on discharge consistency (as measured using an instantaneous mass flow meter) was examined.

Consistency of Discharge

Comparison of the data obtained at 80% apertures for core flow and mass flow (no air), suggests that greater consistency was present in the former test condition. This can be attributed to the flow instability induced by the establishment of the previously documented vena contracta effect (i.e. discharge from the test section was not being controlled by packed bed flow behaviour in the secondary plane flow section). Repeating the mass flow test condition and introducing air gave a significant improvement in consistency of the signal generated—suggesting that the bulk behaviour of the particulate had changed sufficiently for packed bed discharge characteristics to dominate. It is interesting to note that the signal generated for all of the trials that introduced air, appeared to exhibit increased consistency prior to air being positively introduced into the system.

It is considered by the inventor that this change in consistency can be attributed to the fact that although the air was not positively introduced until after approximately 40 seconds into the trial, the partial vacuum in the voids of the bulk particulate was being dissipated by air drawn through the air introduction piping (this was not positively sealed off when not in use).

Results

The results obtained have substantiated the basic premise that considerable and worthwhile improvements in discharge consistency can be obtained through adjustment of geometry and augmentation with the introduction of air.

The low volumes of air introduced into the bulk particulate and the results obtained would seem to suggest that in addition to boosted discharge rates, the correct proportioning of air could yield energy savings at the same time by virtue of reduced compressed air consumption. Current industrial practice does not usually consider the characteristics of the material to be discharged and therefore many systems that use air as a discharge aid (road tanker loading being a particular example) tend to try to excessively aerate the material in the region of the convergent in order to obtain a fast fill rate. The nett result of this practice is invariably excessive fugitive particle entrainment in displaced or vented air from the system being filled. If the use of 3% air by volume in conjunction with geometry favourable to concentrated regions of air induction can provide the levels of increased discharge rate demonstrated in this limited programme of work, then the opinion that improvements bulk flow rate can be obtained without excess dust generation (or associated spillage) is well founded.

The investigations reported above suggest that the application of double plane flow geometry combined with low level aeration has the potential to give very high discharge rates within a compact design. 

1. A method of transferring particulates from a storage silo to another container or to a process, in which the particulates are fed from the silo into a transfer device comprising a conduit having a first convergent section converging in the direction of particulate flow followed by a second convergent section converging in the direction of particulate flow, and the convergence of the second convergent section or the first and second convergent sections is arranged so that the particulates passing though the conduit under gravity are discharged from the second convergent section in a vena contracta flow pattern.
 2. A method according to claim 1 in which air is injected into the second convergent section in a controlled amount that assists formation of the vena contracta.
 3. A method according to claim 1 in which the first convergent section includes first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of the second convergent section, and in the first convergent section the first pair of opposed wall surfaces converge towards each other in the direction of flow, the second pair being convergent, parallel or divergent, whereas in the second convergent section the second pair of opposed wall surfaces converge towards each other in the direction of flow, the first pair being convergent, parallel or divergent.
 4. A method according to claim 1 in which the first convergent section includes at least two opposed wall surfaces which converge towards each other in the direction of flow to form a slot outlet at the junction between the first convergent section and the second convergent section, and the second convergent section includes at least two opposed wall surfaces which converge towards each other in the direction of flow to form a slot outlet for discharge of particulates, the two slot outlets extending in respective directions that are substantially normal to each other.
 5. A method according to claim 3, in which the first convergent section of the conduit converges to form a slot outlet at the junction between the first convergent section and the second convergent section, and the second convergent section converges to form a slot outlet for discharge of particulates, the two slot outlets extending in respective directions that are substantially normal to each other.
 6. A method according to claim 1 in which the conduit includes an inlet section upstream of the first convergent section.
 7. A method according to claim 6 in which the inlet section has parallel walls.
 8. A method according to claim 1 in which the conduit includes a non-convergent section between the first convergent section and the second convergent section.
 9. A method according to claim 1 in which the geometry of the second convergent section is arranged so that the flow discharge of the intended particulates takes the form of a vena contracta.
 10. Device for gravity transfer of bulk particulates comprising an open-ended conduit having a first convergent section and a second convergent section, in which in use particulates pass from the first convergent section to the second convergent section, the first convergent section including first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of the second convergent section, and in the first convergent section the first pair of opposed wall surfaces of the first convergent section converge towards each other in the direction of flow, the second pair being convergent, parallel or divergent, whereas in the second convergent section the second pair of opposed wall surfaces of the second convergent section converge towards each other in the direction of flow, the first pair being convergent, parallel or divergent, in which the convergence of the second section or the first and second sections is arranged so that in use particulates are discharged in a vena contracta flow pattern.
 11. Device according to claim 10, in which the first convergent section of the conduit converges to form a slot outlet at the junction between the first convergent section and the second convergent section, and the second convergent section converges to form a slot outlet for discharge of particulates from the second convergent section, the two slot outlets extending in respective directions that are substantially normal to each other.
 12. Device for gravity transfer of bulk particulates comprising an open-ended conduit having a first convergent section and a second convergent section, in which in use particulates pass from the inlet section to the second convergent section, in which in the first convergent section at least two opposed wall surfaces converge towards each other in the direction of flow to form a slot outlet at the junction between the first convergent section and the second convergent section, and in the second convergent section at least two opposed wall surfaces converge towards each other in the direction of flow to form a slot outlet for discharge of particulates from the second convergent section, the two slot outlets extending in respective directions that are substantially normal to each other, and in which the convergence of the second section or the first and second sections is arranged so that in use particulates are discharged in a vena contracta flow pattern.
 13. Device according to claim 10 in which the first convergent section has first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of the second convergent section, and in the first convergent section the first pair of opposed wall surfaces of the first convergent section converge towards each other in the direction of flow and the second pair are substantially parallel, and in the second convergent section the second pair of opposed wall surfaces of the second convergent section converge towards each other in the direction of flow and the first pair are parallel.
 14. Device according to claim 10 in which the area of the outlet of the second convergent section is from 20-80% of the area of the inlet of the second convergent section.
 15. Device according to claim 10 including means for injecting air into the conduit.
 16. Device according to claim 10 in which the conduit includes an inlet section upstream of the first convergent section.
 17. Device according to claim 16 in which the walls of the inlet section are parallel.
 18. Device according to claim 10 in which the conduit includes a non-convergent section between the first convergent section and the second convergent section.
 19. Device according to claim 11 in which the first convergent section has first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of the second convergent section, and in the first convergent section the first pair of opposed wall surfaces of the first convergent section converge towards each other in the direction of flow and the second pair are substantially parallel, and in the second convergent section the second pair of opposed wall surfaces of the second convergent section converge towards each other in the direction of flow and the first pair are parallel.
 20. Device according to claim 12 in which the first convergent section has first and second pairs of opposed wall surfaces which connect with respective first and second pairs of opposed wall surfaces of the second convergent section, and in the first convergent section the first pair of opposed wall surfaces of the first convergent section converge towards each other in the direction of flow and the second pair are substantially parallel, and in the second convergent section the second pair of opposed wall surfaces of the second convergent section converge towards each other in the direction of flow and the first pair are parallel. 